Tapping of small amounts of CSF from cisterna magna of Mongolian gerbils was carried out 2 hours before subjecting the animals to bilateral occlusion of the common carotid arteries for 15 minutes. Immediately before the clamping of the arteries, the gerbils were given 2 ml of distilled water intraperitoneally. Mortality of these animals during one month observation was compared with that in other groups consisting of a) gerbils subjected to bilateral caortid occlusion alone, b) gerbils subjected to occlusion and administration of distilled water and c) gerbils in which CSF tapping was performed 2 hours before occlusion, but H2O was not given. The group of gerbils subjected to CSF tapping and H2O injection in addition to carotid occlusion showed 85% survival rate after 30 days of clip release. This compared with 60% survival rate of animals subjected to tapping before occlusion. The survival rate of gerbils subjected to occlusion alone was 33%, whereas in animals which in addition received water intraperitoneally the survival rate amounted only to 15%.